How to raycast a list of heights with ray cast
Object.ray_cast takes three parameters: origin, direction and an optional maximum distance.

The parameters origin and direction should both be lists/tuples of length 3 or mathutils.Vector, e.g.

sol = obj.ray_cast((posX, posY, posZ), (0, 0, 1))
location = sol[1]


If you're not sure about whether the original surface is below or above the target, you might want to offset the origin in the function, e.g.

origin = Vector((posX, posY, posZ))
direction = Vector((0, 0, 1))
sol = obj.ray_cast(origin - direction * 100000, direction)
location = sol[1]


Additionally, you have to keep in mind that ray_cast uses local space coordinates. If your target mesh is translated or rotated, you have to convert origin and direction from world space to the target's coordinate system first, and maybe convert the result vector back to world space afterwards. Or you temporarily create a copy of the object with transformations applied so that world space and local space are the same and then cast with the temporary object.